Hot forming aluminum alloy plate and production method therefor

ABSTRACT

The present invention provides an Al—Mg—Si-based hot forming aluminum alloy plate which has not only high age-hardening property but also a high m value in a high strain rate range and excellent surface properties after forming and which is suitable for hot forming. The hot forming aluminum alloy plate comprises an aluminum alloy comprising 0.3 to 1.8 mass % Mg, 0.6 to 2.0 mass % Si and 0.04 to 0.20 mass % Fe. In the aluminum alloy, Mn content is restricted to 0.030 mass % or less, and Cr content is restricted to 0.030 mass % or less, and a balance comprises Al and unavoidable impurities. The hot forming aluminum alloy plate has an electrical conductivity of 60% or less according to IACS %. A production method of the hot forming aluminum alloy plate is also provided.

This is a divisional of application Ser. No. 15/442,128, filed Feb. 24, 2017.

BACKGROUND OF THE INVENTION

The present invention relates to an Al—Mg—Si-based hot forming aluminum alloy plate which has not only high age-hardening property but also a high m value in a high strain rate range and which is suitable for hot forming and to a production method thereof.

Applications of aluminum alloys have been advanced recently as one of measures to reduce the weights of structural parts. However, aluminum alloys generally have poor formability as compared to steel plates, and it is necessary to consider various processing methods. One of the processing methods is hot forming utilizing superplastic deformation. A typical example of such hot forming is blow molding.

Blow molding is a forming method which utilizes the property of aluminum especially, namely its extremely high ductility at a high temperature called superplasticity. Specifically, in a general method, an aluminum plate material is held between upper and lower molds which are heated, and after heating, pressure is applied with high-pressure gas to form the aluminum plate material into the shape of the forming molds. Blow molding utilizes high ductility of an aluminum material at a high temperature and thus enables forming of a complicated shape, which is impossible to achieve by cold press forming. Moreover, because the deformation resistance at a high temperature is small, blow molding has an excellent transfer to the molds and thus is suitable for forming a part that requires excellent design. In addition, because a material can be formed basically using only one of the mold, the cost of the mold can be reduced as compared to cold press forming, and blow molding is used for forming of small quantity and large variety.

With respect to aluminum alloys in particular, materials exhibiting excellent superplastic characteristics are developed actively. In particular, because Al—Cu-based and Al—Zn—Mg—Cu-based aluminum alloys exhibit extremely high ductility at a high temperature and high strength can be obtained by heat treatment after blow molding, some alloys for blow molding have been developed.

However, Al—Cu-based and Al—Zn—Mg—Cu-based aluminum alloys have poor corrosion resistance and poor weldability, and the production costs are high. Thus, applications thereof are limited to special parts of aircrafts and the like at a present state. Here, Al—Mg-based alloys in which a large amount of Mg is in a solid solution state, of course, exhibit high ductility at a high temperature and have moderate strength, moderate weldability and excellent corrosion resistance. Thus, the Al—Mg-based alloys are widely used as hot forming materials for general parts. In particular, applications for automotive parts account for most of the demand thereof. However, as the demand for lighter automotive parts grows, it becomes that hot forming materials having high strength used for general parts is required.

Accordingly, Al—Mg—Si-based alloys for hot forming such as those described in Patent Documents 1 and 2 have been developed recently. However, the formability of the Al—Mg—Si-based alloys for hot forming has not necessarily reached a satisfactory level. In view of the productivity in particular, the m values (the strain rate sensitivity exponent) at 10⁻² to 10⁻¹/second, which is the practical strain rate range, have not been sufficient. An m value is an indication of the resistance to the localization of deformation of a material. Because the Al—Mg—Si-based alloys for hot forming described in the Patent Documents have low m values, deformation is apt to be localized, and it is difficult to form an article which is difficult to form, at a high speed.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2006-37139

Patent Document 2: JP-A-2008-62255

OBJECT AND SUMMARY OF THE INVENTION

The invention has been made to solve the above problems, and an object of the invention is to provide an Al—Mg—Si-based hot forming aluminum alloy plate which has not only high age-hardening property but also a high m value in a high strain rate range and excellent surface properties after forming and which is suitable for hot forming.

The present inventors have extensively investigated the relation among the m value, the alloy components and the electrical conductivity to solve the problems. As a result, the inventors have found that the m value of an Al—Mg—Si-based alloy is improved when Mg is added and when the amounts of Mn and Cr are reduced as low as possible. That is, the inventors have made the following findings. Addition of Mn and Cr limits an improvement of in the m value because Mn- and Cr-based precipitates reduce movable dislocations, while Mg in a solid solution state improves the m value by interacting with dislocations (Solute Drag Creep). Therefore, the inventors have found that it is important to regulate the amount of Mg in a solid solution state and the amounts of Mn and Cr precipitates, and by reducing the electrical conductivity, which is an indication thereof, the m value is improved.

On the other hand, when the added amounts of Mn and Cr are low, the amounts of Mn and Cr precipitates, which stabilize the crystal grain boundaries, decrease. Thus, the crystal grains become coarse, and a rough skin in the surface after forming is apt to occur. Upon investigation to solve this problem, the inventors have found that addition of a certain amount of Fe inhibits decreasing of the m value and is effective in preventing a rough skin in the surface. Based on this finding, the amount of Fe has been specified. As a result, not only high age-hardening property but also improved formability in a high strain rate range can be obtained. The invention has been thus completed.

One aspect of the present invention is that a hot forming aluminum alloy plate comprising an aluminum alloy, wherein the aluminum alloy comprises 0.3 to 1.8 mass % Mg, 0.6 to 2.0 mass % Si and 0.04 to 0.20 mass % Fe, Mn content is restricted to 0.030 mass % or less, Cr content is restricted to 0.030 mass % or less, and a balance comprises Al and unavoidable impurities, and the hot forming aluminum alloy plate has an electrical conductivity of 60% or less according to IACS %.

Further aspect of the present invention is that the aluminum alloy further comprises 0.2 to 1.0 mass % Cu.

Still further aspect of the present invention is that the hot forming aluminum alloy plate is used for blow molding.

Another aspect of the present invention is that a method for producing the hot forming aluminum alloy plate according to any one of claims 1 to 3, the method comprising: a casting step for casting a molten metal of the aluminum alloy; a homogenization step for homogenizing the cast slab; a hot rolling step for hot rolling the homogenized slab; and a cold rolling step for cold rolling the hot-rolled plate: wherein in the homogenization step, the slab is heated and held at a temperature which is 500° C. or higher and lower than the melting point of the aluminum alloy for 1 to 12 hours, and the cooling rate from the completion of heating and holding to 300° C. is 50° C./hour or more; a temperature of the rolled plate during hot rolling is 250 to 450° C. in the hot rolling step; and a reduction ratio in the cold rolling step is 50% or more.

Still another aspect of the present invention is that a method for producing the hot forming aluminum alloy plate according to any one of claims 1 to 3, the method comprising: a casting step for casting a molten metal of the aluminum alloy; a homogenization step for homogenizing the cast slab; a hot rolling step for hot rolling the homogenized slab; a cold rolling step for cold rolling the hot-rolled plate; and an annealing step for annealing the rolled plate: wherein in the homogenization step, the slab is heated and held at a temperature which is 500° C. or higher and lower than the melting point of the aluminum alloy for 1 to 12 hours, and the cooling rate from the completion of heating and holding to 300° C. is 50° C./hour or more; the temperature of the rolled plate during hot rolling is 250 to 450° C. in the hot rolling step; the reduction ratio in the cold rolling step is 50% or more; and in the annealing step, the rolled plate is annealed at a temperature of 500 to 580° C. during the cold rolling step, the heating rate to the annealing temperature is 5° C./second or more, and the cooling rate after annealing is 100° C./second or more.

Further aspect of the present invention is that the reduction ratio in the cold rolling step is 80% or more.

Still further aspect of the present invention is that a DC casting process in which the cooling rate is 50° C./minute or more is used in the casting step.

According to the invention, an Al—Mg—Si-based hot forming aluminum alloy plate which has not only high age-hardening property but also a high m value in a high strain rate range and excellent surface properties after forming and which is suitable for hot forming is obtained.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, to improve the m value of the aluminum alloy, the precipitation of second phase particles as the metallic structure is inhibited in relation to the electrical conductivity. Also, the average crystal grain size is specified to inhibit the rough skin in the surface after forming. Moreover, the tensile strength after aging, which is required for applications for general parts, is also specified. The alloy composition to obtain these characteristics is also specified. Each of the items for the aluminum alloy plate for hot forming of the present invention is explained in detail below.

1. Metallic Structure

1-1. Second Phase Particles

Mn-based and Cr-based second phase particles especially inhibit movable dislocations and decrease the m value. Accordingly, in the invention, the amount of formed second phase particles such as Mn-based, Cr-based and Mg—Si-based second phase particles (the amount of those which are not in a solid solution state but are precipitated, hereinafter referred to as “the amount of precipitated second phase particles”) is inhibited. The amount of the precipitated second phase particles can be estimated by the electrical conductivity of the aluminum alloy. In general, as the electrical conductivity of the aluminum alloy becomes higher, the amount of second phase particles in a solid solution becomes lower, namely the amount of precipitated second phase particles becomes higher. In the present invention, the electrical conductivity of the aluminum alloy according to IACS % is 60% or less, preferably 58% or less. When the IACS % is 60% or less, the amount of precipitated second phase particles is inhibited, and it is expected that the m value is improved. The lower limit of the electrical conductivity is not particularly limited but is approximately 56% in the present invention due to the aluminum alloy composition and the production method.

1-2. Average Crystal Grain Size

When the crystal grain size of the aluminum alloy is large, the rough skin in the surface after hot forming occurs. According to the investigation by the inventors, the rough skin in the surface after forming can be inhibited effectively when the average crystal grain size just before hot forming is 50 μm or less, preferably 45 μm or less. The lower limit of the average crystal grain size is not particularly limited but is approximately 40 μm in the present invention due to the aluminum alloy composition and the production method. In this regard, the average crystal grain size of the crystal structure was determined by measuring the average crystal grain size of crystal grains surrounded by grain boundaries at high angles of 15° or more in a field of 800 μm×1600 μm using EBSD (Electron Backscatter Diffraction).

2. The m Value

In the present invention, by inhibiting the amount of precipitated second phase particles such as Mn-based, Cr-based and Mg—Si-based second phase particles, the m value at 10⁻² to 10⁻¹/second, which is the practical strain rate range, is set at 0.23 or more, preferably 0.25 or more. When the m value is less than 0.23, the deformation is localized during hot forming, and the properties for hot forming deteriorate. The upper limit of them value is not particularly limited but is approximately 0.29 in the present invention due to the aluminum alloy composition and the production method.

3. Tensile Strength After Aging

The hot forming aluminum alloy plate according to the present invention has a tensile strength of 300 MPa or more, preferably 315 MPa or more, which is sufficient for applications for general parts, as the tensile strength after aging after hot forming. The upper limit of the tensile strength after aging is not particularly limited but is approximately 330 MPa in the invention due to the aluminum alloy composition and the production method.

4. Component Composition of Aluminum Alloy

With respect to the component composition of the hot forming aluminum alloy plate of the invention, Mg, Si and Fe are essential elements, and the Mn and Cr contents are restricted. Also, Cu is an optional element. The reasons for the limitations are given below.

4-1. Mg: 0.3 to 1.8 Mass % and Si: 0.6 to 2.0 Mass %

Mg and Si are basic elements of the aluminum alloy used in the present invention. These both elements are essential additive elements for securing the properties for superplastic forming and for obtaining high strength which is equivalent to or higher than that of an Al—Mg-based aluminum alloy by age hardening treatment after hot forming. Also, Mg in a solid solution causes Solute Drag Creep by interacting with dislocations during deformation at a high temperature and improves the m value. Thus, it is necessary to add a certain amount of Mg. When the Mg content is less than 0.3 mass % (hereinafter simply referred to as “%”) or when the Si content is less than 0.6%, the above effects are not obtained sufficiently. On the other hand, when the Mg content exceeds 1.8% or when the Si content exceeds 2.0%, an Mg—Si-based second phase is formed, resulting in a decrease in the m value. Accordingly, the Mg content is specified to be 0.3 to 1.8%, and the Si content is specified to be 0.6 to 2.0%. The Mg content is preferably 0.6 to 1.4%, and the Si content is preferably 0.8 to 1.4%.

4-2. Fe: 0.04 to 0.20%

Although Fe-based precipitates are formed by adding Fe, a necessary amount of Fe is added to inhibit decreasing of the m value and at the same time to inhibit the rough skin in the surface by stabilizing the crystal grains. When the Fe content is less than 0.04%, the crystal grains cannot be stabilized, and the rough skin in the surface occurs. Moreover, use of high purity metal is required, resulting in an increase in the raw material costs. On the other hand, when the Fe content exceeds 0.20%, a sufficient m value is not obtained. The Fe content is preferably 0.08% to 0.14%.

4-3. Mn: 0.030% or Less and Cr: 0.030% or Less

By adding Mn and Cr, Mn-based precipitates and Cr-based precipitates are formed, and movable dislocations are thus inhibited. As a result, the effect of Solute Drag Creep is inhibited, and the m value decreases. Accordingly, the Mn and Cr contents are each restricted to 0.030% or less. When the Mn content exceeds 0.030% or when the Cr content exceeds 0.030%, a sufficient m value is not obtained. The Mn content is preferably 0.010% or less, and the Cr content is preferably 0.010% or less. The Mn content and the Cr content may be 0%.

4-4. Cu: 0.2 to 1.0%

Cu may be added optionally according to the need because Cu improves the age-hardening property. When the Cu content is less than 0.2%, a sufficient effect of the addition is not obtained. On the other hand, when the Cu content exceeds 1.0%, the corrosion resistance decreases. Accordingly, the Cu content is preferably 0.2 to 1.0%, more preferably 0.3 to 0.7%.

4-6. Ti: 0.20% or Less

Ti may be added optionally according to the need because the slab structure can be made fine by adding Ti. However, the corrosion resistance decreases when Ti is added. No problem arises with the effects of the present invention when the Ti content is 0.20% or less.

4-7. Unavoidable Impurities

It is acceptable that Zr, Zn, B, Be and the like are contained as unavoidable impurities each in an amount of 0.05% or less and in a total amount of 0.15% or less because the effects of the present invention are not impaired.

5. Production Method

Next, the method for producing the hot forming aluminum alloy plate according to the present invention is explained.

5-1. Melting and Casting Step

First, a molten alloy metal having the alloy composition is prepared and cast it. Casting is conducted by a general method such as DC casting. At this point, it is preferable to inhibit the formation of coarse second phase particles by increasing the cooling rate. In the present invention, the cooling rate in DC casting (Direct Chill Casting) is preferably 50° C./minute or more, more preferably 100° C./minute or more. The upper limit of the cooling rate is not particularly limited but is approximately 300° C./minute in the present invention due to the production method and the production apparatus used.

5-2. Homogenization Step

The slab of the aluminum alloy obtained by melting and casting is subjected to a homogenization step after facing the slab. The homogenization temperature is specified to be 500° C. or higher and lower than the melting point of the aluminum alloy used in the present invention (for example, approximately 580° C.) When the heating temperature is lower than 500° C., the effect of improving the m value that is obtained when second phase particles which decrease the m value, dissolve again into a solid solution, is not obtained. When the homogenization temperature is lower than the melting point of the aluminum alloy used in the present invention, the aluminum alloy can be prevented from melting. Accordingly, the homogenization temperature is 500° C. or higher and lower than the melting point of the aluminum alloy, preferably 530 to 560° C. The homogenization period (heating and holding period) is preferably 1 to 12 hours, more preferably 2 to 8 hours. When the period is shorter than one hour, redissolution of second phase particles which decrease the m value, into a solid solution is not promoted, while when the period exceeds 12 hours, Fe which is in a solid solution in supersaturated state during casting is precipitated as a compound, resulting in coarse crystal grains after forming. The cooling rate after the homogenization step (completion of heating and holding) to 300° C. is 50° C./hour or more, preferably 100° C./hour or more. When the cooling rate is 50° C./hour or more, precipitation of coarse second phase particles which decrease the m value, is inhibited. The upper limit of the cooling rate is not particularly limited but is approximately 360° C./hour in the present invention due to the production method and the production apparatus used.

5-3. Hot Rolling Step

The temperature of the material during hot rolling is 250 to 450° C., preferably 350 to 400° C. When the temperature is 250° C. or higher, the deformation resistance of the material becomes small, and thus hot rolling becomes easy. On the other hand, when the temperature is 450° C. or lower, precipitation of coarse second phase particles is inhibited during hot rolling. As a result, the m value increases, and the strength after aging after hot forming is improved.

5-4. Cold Rolling Step

In the present invention, the rolled plate after the hot rolling step is subjected to a cold rolling step, and then the cold-rolled plate can be subjected directly to hot forming such as hot blow molding. When the reduction ratio in the cold rolling step is large, the crystal grains after final annealing become fine, and the effect of inhibiting the rough skin in the surface is exhibited. The reduction ratio is 50% or more, preferably 80% or more. The upper limit of the reduction ratio is not particularly limited but is approximately 95% in the invention due to the alloy composition, the production method, the rolling apparatus and the like.

5-5. Annealing Step

To dissolve second phase particles again into a solid solution, an annealing step for annealing the rolled plate may be provided during the cold rolling step. Increasing the amount of second phase particles in a solid solution by annealing and conducting cold rolling in this state are more effective in making the crystal grains fine, and thus the effect of inhibiting the rough skin in the surface is exhibited. The annealing temperature is 500 to 580° C., preferably 530 to 570° C. When the annealing temperature is 500° C. or higher, the amount of second phase particles in a solid solution can be increased. However, when the annealing temperature exceeds 580° C., the material melts partially, resulting in deterioration of the formability. The heating rate to the annealing temperature is 5° C./second or more. When the heating rate is less than 5° C./second, second phase particles are precipitated during temperature rising, and the m value decreases. Moreover, the strength after aging after hot forming decreases. The upper limit of the heating rate is not particularly limited but is approximately 10° C./second in the invention due to the production method and the production apparatus used. Furthermore, it is preferable that the cooling rate after annealing to room temperature is 100° C./second or more. When the cooling rate is less than 100° C./second, second phase particles are precipitated during cooling, and the m value decreases. Moreover, the strength after aging after hot forming decreases. The upper limit of the cooling rate is not particularly limited but is approximately 400° C./second in the present invention due to the production method and the production apparatus used.

EXAMPLES

Examples of the present invention are explained below. The aluminum alloys (alloy numbers 1 to 24) described in Tables 1 and 5 were each melted and cast by the DC casting process. The cooling rate in DC casting was 80° C./minute. After facing the obtained slabs, the slabs were homogenized and then cooled under the respective conditions in Table 2. Subsequently, the slabs were hot rolled with the temperatures of the rolled plates during rolling set at the respective temperatures in Table 2. At the end, the rolled plates after hot rolling were subjected to process annealing and cold rolling under the respective conditions in Table 2, and thus rolled plate samples having a final thickness of 1 mm were obtained. Process annealing was conducted by using a salt bath.

TABLE 1 Alloy Composition (mass %) Al and Alloy Unavoidable Number Mg Si Fe Mn Cr Ti Impurities Remarks 1 1.0 1.4 0.08 0.005 0.005 0.01 balance within the 2 0.5 1.4 0.08 0.005 0.005 0.01 balance scope of 3 1.6 1.4 0.08 0.005 0.005 0.01 balance the 4 1.0 0.7 0.08 0.005 0.005 0.01 balance invention 5 1.0 1.8 0.08 0.005 0.005 0.01 balance 6 1.0 1.4 0.08 0.005 0.005 0.01 balance 7 1.0 1.4 0.18 0.005 0.005 0.01 balance 8 1.0 1.4 0.06 0.005 0.005 0.01 balance 9 1.0 1.4 0.08 0.012 0.005 0.01 balance 10 1.0 1.4 0.08 0.025 0.005 0.01 balance 11 1.0 1.4 0.08 0.005 0.012 0.01 balance 12 1.0 1.4 0.08 0.005 0.025 0.01 balance 13 0.2 1.4 0.08 0.005 0.005 0.01 balance outside the 14 2.0 1.4 0.08 0.005 0.005 0.01 balance scope of 15 1.0 0.4 0.08 0.005 0.005 0.01 balance the 16 1.0 2.1 0.08 0.005 0.005 0.01 balance invention 17 1.0 1.4 0.02 0.005 0.005 0.01 balance 18 1.0 1.4 0.22 0.005 0.005 0.01 balance 19 1.0 1.4 0.08 0.035 0.005 0.01 balance 20 1.0 1.4 0.08 0.005 0.032 0.01 balance

TABLE 2 Hot Rolling Step Temper- Final ature of Process Annealing Step Cold Casting Rolled Temper- Rolling Step Homogenization Step Plate ature Step Cooling Cooling During raising Cooling Re- Pro- Rate Temper- Rate Hot Rate Temper- Rate duction duction ° C./ ature Period (° C./ Rolling (° C./ ature (° C./ ratio Number minute (° C.) (hour) hour) (° C.) second) (° C.) second) (%) 1 80 550 6 60 370 — not conducted — 84 2 80 520 6 60 370 — not conducted — 84 3 80 550 10 60 370 — not conducted — 84 4 80 550 4 60 370 — not conducted — 84 5 80 550 1 60 370 — not conducted — 84 6 80 550 6 60 420 — not conducted — 84 7 80 550 6 60 370 — not conducted — 84 8 80 550 6 60 370 — not conducted — 84 9 80 550 6 60 370 — not conducted — 84 10 80 550 6 60 370 — not conducted — 84 11 80 550 6 60 370 — not conducted — 84 12 80 550 6 60 370 10 550 120 84 13 80 550 6 60 370 10 510 120 84 14 80 550 6 60 370 10 575 120 84 15 80 550 6 120 370 — not conducted — 84 16 80 550 6 360 370 — not conducted — 84 17 80 550 6 60 370 — not conducted — 66 18 80 550 6 360 370 — not conducted — 84 19 160 550 6 60 370 — not conducted — 84 20 300 550 6 60 370 — not conducted — 84 21 80 520 6 60 370 10 550 400 84 22 80 480 6 60 370 — not conducted — 84 23 80 590 6 60 370 — not conducted — 84 24 80 550 14 60 370 — not conducted — 84 25 80 550 0.5 60 370 — not conducted — 84 26 80 550 6 40 370 — not conducted — 84 27 80 550 6 60 200 — not conducted — 84 28 80 550 6 60 470 — not conducted — 84 29 80 550 6 60 370 — not conducted — 45 30 80 550 6 60 370 10 460 120 84 31 80 550 6 60 370 10 590 120 84 32 80 550 6 60 370 0.01 550 120 84 33 80 550 6 60 370 10 550 40 84

TABLE 5 Alloy Composition (mass %) Al and Alloy Unavoidable Number Mg Si Fe Mn Cr Cu Ti Impurities 21 1.0 1.4 0.08 0.005 0.005 0.3 0.01 balance within the scope of the invention 22 1.0 1.4 0.08 0.005 0.005 0.8 0.01 balance within the scope of the invention 23 1.0 1.4 0.08 0.005 0.005 1.2 0.01 balance outside the scope of the invention 24 1.0 1.4 0.08 0.005 0.005 0.3 0.20 balance within the scope of the invention

6. Evaluation of Samples

6-1. Electrical Conductivity IACS %

A sample was cut into a piece of 100 mm×100 mm, and the IACS % of the sample was measured using a Sigma tester. The measurement was conducted five times, and the electrical conductivity of the sample was determined by the arithmetic mean.

6-2. The m Value

A sample was processed into a high-temperature tensile test piece. The piece was placed on a high-temperature tensile tester, and then the m value was measured by the strain-rate jump test. The temperature of the tensile test was 530° C. The plot of stress-strain rate at 10⁻² to 10⁻¹/second was linearly approximated, and the slope of the line was used as them value. Samples having an m value of 0.23 or more were determined to be acceptable, and samples having an m value of less than 0.23 were determined to be unacceptable.

6-3. Average Crystal Grain Size After Annealing

After heating (annealing) a sample at 530° C. for five minutes, the crystal grains of a cross section of the sample were observed by EBSP, and the crystal grain sizes were measured. Grain boundaries at high angles of 15° or more were regarded as crystal grain boundaries, and the sizes of the crystal grains were measured. Specifically, the sizes of crystal grains surrounded by grain boundaries at high angles of 15° or more in a field of 800 μm×1600 μm were measured using EBSP, and the average crystal grain size was determined by the arithmetic mean thereof. Samples having an average crystal grain size of 50 μm or less were determined to be acceptable, and samples having an average crystal grain size of more than 50 μm were determined to be unacceptable.

6-4. Tensile Strength After Aging

Three test pieces of 3 cm×20 cm were cut out of a sample and subjected to heat treatment at 530° C. for one hour, which simulated high-temperature forming. The test pieces were water cooled to room temperature for quenching treatment and subsequently subjected to batch aging treatment of 180° C.×one hour. The tensile strengths of the test pieces after batch aging treatment were measured in accordance with the JIS No. 5 tensile test. The tensile strength after aging after hot forming was determined by the arithmetic mean of the values of the test pieces. Samples having a tensile strength of 300 MPa or more were determined to be acceptable, and samples having a tensile strength of less than 300 MPa were determined to be unacceptable.

6-5. Evaluation of Corrosion Resistance

Three test pieces of 5 cm×6 cm were cut out of a sample having chemical components of Table 5 and subjected to heat treatment at 530° C. for one hour, which simulated high-temperature forming. The test pieces were water cooled to room temperature for quenching treatment and subsequently subjected to batch aging treatment of 180° C.×one hour, and a grain boundary corrosion test was conducted based on the standard of ISO11846 (b). The corrosion resistance was determined to be acceptable (A) when the corrosion depth was less than 300 μm, and the corrosion resistance was determined to be unacceptable (B) when the corrosion depth was 300 μm or more.

The results of the evaluations above are shown in Tables 3, 4 and 6. Table 3 includes the results of samples which were produced under the same production conditions and which had different alloy compositions. Table 4 includes the results of samples which had the same alloy composition and which were produced under different production conditions. Table 6 includes the results of samples to which optionally additive elements of the aluminum alloy were added. The overall evaluations of samples were determined to be acceptable (A) when the m value, the average crystal grain size and the tensile strength after aging were all acceptable, and the overall evaluations of the remaining samples were determined to be unacceptable (C).

TABLE 3 Average Tensile Pro- The Crystal Strength Over- Alloy duction m Grain After all Num- Num- IACS Value Size Aging Eval- ber ber (%) (—) (μm) (MPa) uation Example 1 1 1 55 0.26 45 320 A Example 2 2 1 57 0.25 46 307 A Example 3 3 1 58 0.23 41 333 A Example 4 4 1 57 0.25 42 312 A Example 5 5 1 58 0.24 46 326 A Example 6 6 1 56 0.26 48 318 A Example 7 7 1 57 0.24 35 325 A Example 8 8 1 56 0.23 38 328 A Example 9 9 1 58 0.24 42 322 A Example 10 10 1 57 0.23 40 325 A Example 11 11 1 58 0.24 42 321 A Example 12 12 1 59 0.23 39 325 A Comparative 13 1 61 0.21 48 279 C Example 1 Comparative 14 1 53 0.21 39 350 C Example 2 Comparative 15 1 62 0.22 38 243 C Example 3 Comparative 16 1 55 0.21 37 330 C Example 4 Comparative 17 1 61 0.26 54 315 C Example 5 Comparative 18 1 52 0.21 32 329 C Example 6 Comparative 19 1 57 0.21 39 325 C Example 7 Comparative 20 1 57 0.21 36 326 C Example 8

TABLE 4 Average Tensile Pro- The Crystal Strength Alloy duction m Grain After Overall Num- Num- IACS Value Size Aging Eval- ber ber (%) (—) (μm) (MPa) uation Example 13 1 1 57 0.25 46 307 A Example 14 1 2 54 0.24 43 307 A Example 15 1 3 52 0.25 41 322 A Example 16 1 4 53 0.25 42 321 A Example 17 1 5 55 0.25 43 316 A Example 18 1 6 54 0.23 43 306 A Example 19 1 7 54 0.23 43 306 A Example 20 1 8 52 0.25 43 317 A Example 21 1 9 54 0.23 40 321 A Example 22 1 10 58 0.23 35 322 A Example 23 1 11 59 0.25 44 323 A Example 24 1 12 57 0.27 44 340 A Example 25 1 13 58 0.26 42 330 A Example 26 1 14 57 0.26 41 321 A Example 27 1 15 58 0.26 43 332 A Example 28 1 16 57 0.27 43 341 A Example 29 1 17 59 0.24 49 318 A Example 30 1 18 54 0.26 46 325 A Example 31 1 19 54 0.27 44 320 A Example 32 1 20 53 0.27 44 332 A Example 33 1 21 54 0.26 46 325 A Comparative 1 22 61 0.22 47 280 C Example 9 Comparative 1 23 51 0.21 44 308 C Example 10 Comparative 1 24 52 0.25 52 322 C Example 11 Comparative 1 25 55 0.22 44 285 C Example 12 Comparative 1 26 58 0.22 48 293 C Example 13 Comparative 1 27 — — — — C Example 14 Comparative 1 28 56 0.22 47 295 C Example 15 Comparative 1 29 51 0.23 55 310 C Example 16 Comparative 1 30 55 0.22 59 297 C Example 17 Comparative 1 31 60 0.21 50 304 C Example 18 Comparative 1 32 62 0.21 49 286 C Example 19 Comparative 1 33 59 0.21 46 295 C Example 20

TABLE 6 Average Tensile Crystal Strength Pro- The m Grain After Overall Overall Alloy duction IACS Value Size Aging Resist- Eval- Number Number (%) (—) (μm) (MPa) ance uation Example 34 21 1 54 0.24 41 328 A A Example 35 22 1 55 0.25 44 342 A A Comparative 23 1 52 0.23 41 350 B A Example 21 Example 36 24 1 55 0.25 44 323 A A

In Table 3, because samples having the alloy compositions specified by the invention were used in Examples 1 to 12, the IACS values were satisfied, and the m values, the crystal grain sizes and the tensile strengths after aging were all acceptable. The overall evaluations were also acceptable.

On the other hand, because the Mg content was too low in Comparative Example 1, the IACS was outside the range specified by the present invention, and the m value and the tensile strength after aging were unacceptable. The overall evaluation was also unacceptable.

Because the Mg content was too high in Comparative Example 2, a second phase was precipitated, and the m value was unacceptable. The overall evaluation was also unacceptable.

Because the Si content was too low in Comparative Example 3, the IACS was outside the range specified by the present invention, and them value and the tensile strength after aging were unacceptable. The overall evaluation was unacceptable.

Because the Si content was too high in Comparative Example. 4, a second phase was precipitated, and the m value was unacceptable. The overall evaluation was also unacceptable.

Because the Fe content was too low in Comparative Example 5, the IACS was outside the range specified by the invention, and the average crystal grain size became coarse. The overall evaluation was unacceptable.

Because the Fe content was too high in Comparative Example 6, a second phase was precipitated, and the m value was unacceptable. The overall evaluation was also unacceptable.

Because the Mn content was too high in Comparative Example 7, a second phase was precipitated, and the m value was unacceptable. The overall evaluation was unacceptable.

Because the Cr content was too high in Comparative Example 8, a second phase was precipitated, and the m value was unacceptable. The overall evaluation was unacceptable.

In Table 4, because the production conditions specified by the present invention were used in Examples 13 to 33, the IACS values were satisfied, and them values, the crystal grain sizes and the tensile strengths after aging were all acceptable. The overall evaluations were also acceptable.

On the other hand, because the homogenization temperature was too low in Comparative Example 9, the IACS was outside the range specified by the present invention, and the m value and the tensile strength after aging were unacceptable. The overall evaluation was also unacceptable.

Because the homogenization temperature was too high in Comparative Example 10, the slab melted during the homogenization. Moreover, a second phase was formed, and the m value decreased. Thus, the overall evaluation was unacceptable.

Because the homogenization period was too long in Comparative Example 11, Fe-based precipitates were formed, and the average crystal grain size became large. Thus, the overall evaluation was also unacceptable.

Because the homogenization period was too short in Comparative Example 12, a second phase remained, and thus the m value and the strength after aging decreased. Thus, the overall evaluation was also unacceptable.

Because the cooling rate after the homogenization step was too slow in Comparative Example 13, a second phase was formed, and the m value and the strength after aging decreased. Thus, the overall evaluation was unacceptable.

Because the temperature of the rolled plate during hot rolling was too low in Comparative Example 14, the deformation resistance during hot rolling became large, and hot rolling was impossible.

Because the temperature of the rolled plate during hot rolling was too high in Comparative Example 15, a second phase was formed, and the m value and the strength after aging decreased. Thus, the overall evaluation was unacceptable.

Because the reduction ratio in the cold rolling step was too small in Comparative Example 16, the crystal grain size became coarse, and the overall evaluation was also unacceptable.

Because the process annealing temperature was too low in Comparative Example 17, a second phase was formed, and the m value decreased. Moreover, the crystal grain size became coarse, and the strength after aging decreased. Thus, the overall evaluation was unacceptable.

Because the process annealing temperature was too high in Comparative Example 18, the eutectic crystal melted during annealing, and the m value decreased. Thus, the overall evaluation was also unacceptable.

Because the temperature raising rate in the process annealing step was too low in Comparative Example 19, the IACS was outside the range specified by the present invention, and a second phase was formed. Moreover, the m value decreased, and the strength after aging decreased. Thus, the overall evaluation was unacceptable.

Because the cooling rate after process annealing was too low in Comparative Example 20, a second phase was formed, and the m value and the strength after aging decreased. Thus, the overall evaluation was unacceptable.

In Table 6, because samples having the alloy compositions specified by the present invention were used in Examples 34 to 36, the IACS values were satisfied, and the m values, the crystal grain sizes, the tensile strengths after aging and the corrosion resistance were all acceptable. The overall evaluations were also acceptable.

On the other hand, because the Cu content was too high, Comparative Example 21 resulted in poor corrosion resistance.

The hot forming aluminum alloy plate according to the present invention has not only high age-hardening property but also a high m value in a high strain rate range and excellent surface properties after forming and thus is excellently industrial applicability. 

What is claimed is:
 1. A method for producing the hot forming aluminum alloy plate wherein the aluminum alloy comprises 0.3 to 1.8 mass % Mg, 0.6 to 2.0 mass % Si and 0.04 to 0.20 mass % Fe, Mn content is restricted to 0.030 mass % or less, Cr content is restricted to 0.030 mass % or less, and a balance comprises Al and unavoidable impurities, and the hot forming aluminum alloy plate has an electrical conductivity of 60% or less according to IACS %, the method comprising: a casting step for casting a molten metal of the aluminum alloy; a homogenization step for homogenizing the cast slab; a hot rolling step for hot rolling the homogenized slab; and a cold rolling step for cold rolling the hot-rolled plate: wherein in the homogenization step, the slab is heated and held at a temperature which is 500° C. or higher and lower than the melting point of the aluminum alloy for 1 to 12 hours, and the cooling rate from the completion of heating and holding to 300° C. is 50° C./hour or more; a temperature of the rolled plate during hot rolling is 250 to 450° C. in the hot rolling step; and a reduction ratio in the cold rolling step is 50% or more.
 2. The method for producing the hot forming aluminum alloy plate according to claim 1, wherein the reduction ratio in the cold rolling step is 80% or more.
 3. The method for producing the hot forming aluminum alloy plate according to claim 1, wherein a DC casting process in which the cooling rate is 50° C./minute or more is used in the casting step.
 4. A method for producing the hot forming aluminum alloy plate wherein the aluminum alloy comprises 0.3 to 1.8 mass % Mg, 0.6 to 2.0 mass % Si and 0.04 to 0.20 mass % Fe, Mn content is restricted to 0.030 mass % or less, Cr content is restricted to 0.030 mass % or less, and a balance comprises Al and unavoidable impurities, and the hot forming aluminum alloy plate has an electrical conductivity of 60% or less according to IACS %, the method comprising: a casting step for casting a molten metal of the aluminum alloy; a homogenization step for homogenizing the cast slab; a hot rolling step for hot rolling the homogenized slab; a cold rolling step for cold rolling the hot-rolled plate; and an annealing step for annealing the rolled plate: wherein in the homogenization step, the slab is heated and held at a temperature which is 500° C. or higher and lower than the melting point of the aluminum alloy for 1 to 12 hours, and the cooling rate from the completion of heating and holding to 300° C. is 50° C./hour or more; the temperature of the rolled plate during hot rolling is 250 to 450° C. in the hot rolling step; the reduction ratio in the cold rolling step is 50% or more; and in the annealing step, the rolled plate is annealed at a temperature of 500 to 580° C. during the cold rolling step, the heating rate to the annealing temperature is 5° C./second or more, and the cooling rate after annealing is 100° C./second or more.
 5. The method for producing the hot forming aluminum alloy plate according to claim 4, wherein the reduction -ratio in the cold rolling step is 80% or more.
 6. The method for producing the hot forming aluminum alloy plate according to claim 4, wherein a DC casting process in which the cooling rate is 50° C./minute or more is used in the casting step. 